A known system for treating exhaust gas passing through an exhaust system of a diesel engine comprises a diesel oxidation catalyst (DOC) that oxidizes hydrocarbons (HC) to CO2 and H2O and converts NO to NO2, and a diesel particulate filter (DPF) that traps diesel particulate matter (DPM). DPM includes soot or carbon, the soluble organic fraction (SOF), and ash (i.e. lube oil additives etc.). The DPF is located downstream of the DOC in the exhaust gas flow. The combination of these two exhaust gas treatment devices prevents significant amounts of pollutants such as hydrocarbons, carbon monoxide, soot, SOF, and ash, from entering the atmosphere. The trapping of DPM by the DPF prevents black smoke from being emitted from a vehicle's exhaust pipe.
A DPF requires regeneration from time to time in order to maintain particulate trapping efficiency. Regeneration involves the presence of conditions that will burn off trapped particulates whose unchecked accumulation would otherwise impair DPF effectiveness. While “regeneration” refers to the general process of burning off DPM, two particular types of regeneration are recognized by those familiar with the regeneration technology as presently being applied to motor vehicle engines.
“Passive regeneration” is generally understood to mean regeneration that can occur anytime that the engine is operating under conditions that burn off DPM without initiating a specific regeneration strategy embodied by algorithms in an engine control system. “Active regeneration” is generally understood to mean regeneration that is initiated intentionally, either by the engine control system on its own initiative or by the driver causing the engine control system to initiate a programmed regeneration strategy, with the goal of elevating temperature of exhaust gases entering the DPF to a range suitable for initiating and maintaining burning of trapped particulates.
The creation of conditions for initiating and continuing active regeneration generally involves elevating the temperature of exhaust gas entering the DPF to a suitably high temperature, such as 500° to 600° C. There are several methods of doing so, such as retarding the start of main fuel injections or post-injection of diesel fuel to elevate exhaust gas temperatures entering the DPF while still leaving excess oxygen for burning the trapped particulate matter. Post-injection of diesel fuel may be used in conjunction with other procedures and/or devices for elevating exhaust gas temperature to the relatively high temperatures needed for active DPF regeneration. One typical method of elevating exhaust gas temperature is by allowing the post-injected diesel fuel to have an exothermic reaction with the oxygen in the DOC, thereby creating the high temperatures needed for regeneration.
As an emission control method, it is known to use exhaust gas recirculation (EGR) to re-circulate exhaust gas back into the cylinders. Exhaust gas recirculation reduces the amount of excess oxygen and lowers the peak combustion temperature of the engine. The EGR gas may be fed through a cooler before it is sent back to the intake manifold.
DPF regeneration methods require extra effort, equipment, and energy. A user or an engine control system must monitor the condition of the DPF in order to initiate the active regeneration. In addition, the engine typically must be fitted with supplemental equipment, such as the apparatus to inject fuel into the exhaust gas. Furthermore, overall efficiency may be reduced because additional fuel must be spent in order to produce heat to burn off the particulates.
The present inventor has recognized the benefit for an engine system that regenerates diesel particulate filters without burning additional fuel, or expending additional effort, or installing additional active generation equipment.